Factors associated with undiagnosed hypertension among Tongan adults: a cross-sectional study

Background Hypertension is responsible for many premature deaths worldwide. However, many individuals with hypertension remain undiagnosed. Tonga is one of the countries that has had a steep increase in hypertension, thus undiagnosed hypertension could also be increasing. Purpose of this study was to assess the prevalence and factors associated with undiagnosed hypertension among Tongan adults. Methods This cross-sectional study used data collected from conveniently sampled 473 participants using electronic questionnaire and digital sphygmomanometer through household visits between February and March 2023. Inclusion criteria were age of 18–65 years, residence in the villages for at least six months, and not being pregnant. Fisher’s exact test and mixed-effect logistic regression were performed using the EZR software to assess the association between undiagnosed hypertension and predictor variables. Results The prevalence of undiagnosed hypertension was 22.4% (106/473). Five variables that were significantly associated with undiagnosed hypertension in Fisher’s exact test were included in the multivariate logistic regression. Overall, only three variables remained significant. First, participants who never had their blood pressure measured had higher prevalence compared to those who had it checked recently (33.3% vs. 19.1%); odds ratio: 2.24). Secondly, participants who were not aware of the risk of developing hypertension were significantly more likely to have undiagnosed hypertension compared to those who were aware (27.9% vs. 16.7%; odds ratio: 1.81). Lastly, middle-aged participants (30–49 years) and older (50–65 years), were significantly more likely to have undiagnosed hypertension compared to those who were 18–29 years old (30.0% and 23.7% vs. 11.8%; odds ratio: 3.58 and 3.38 vs. 1.00). Conclusion The prevalence of undiagnosed hypertension could be substantial among Tongan adults, implicating a need to address this issue by doing further research and review current public health work to address hypertension in Tonga. Undiagnosed hypertension was associated with having no experience of blood pressure measurement, lack of awareness about hypertension, and age. Tongan government should provide people with more opportunities to have their blood pressure measured and to improve their awareness. Supplementary Information The online version contains supplementary material available at 10.1186/s41182-023-00570-4.


Background
Hypertension is one of the major causes of premature death across the globe, and 1.28 billion adults have been diagnosed with high blood pressure [1].According to the World Health Organization (WHO), one of the global targets for non-communicable diseases (NCDs) is to control and decrease the prevalence rate of hypertension by 33% compared to 2010 by the year 2030 [1,2].Two-thirds of people with hypertension live in low-and middleincome countries [3], and it is a major burden in terms of economic development, social relationships, and most importantly, lives.Additionally, about 46% of adults who have hypertension are not diagnosed globally [1].
Undiagnosed hypertension occurs when those who have hypertension have not been informed of the condition by a health professional [4][5][6].It is a silent but deadly condition that can lead to serious health consequences, including heart disease, stroke, kidney disease, and cognitive impairment [1].Despite the availability of effective treatments for hypertension, undiagnosed hypertension remains a major health concern due to its lack of symptoms and low awareness among the general public [4,7].Hence, countries across the globe need to develop locally contextualized strategies to assess, monitor, and regulate the prevalence of undiagnosed hypertension [1,8].According to the first-ever global review of studies on hypertension 2019, 41% of women and 51% of men with hypertension remained undiagnosed in 200 countries, and regions of Oceania and sub-Saharan Africa had the highest prevalence of undiagnosed hypertension [9].
Tonga is one of the Pacific Islands that has had a steep increase in hypertension from 1999 to 2019 [2,3,9].Tonga is an archipelago that is part of Polynesia and is composed of 179 islands.The total population is 100,180, of which 74% reside on the main island of Tongatapu [14].Health care services are predominantly provided by the government with one national referral hospital, three community hospitals, 34 reproductive health clinics, and 14 health centers [14,15].On Tongatapu, there are seven health centers located in rural areas that provide health services for nearby villages [14].They are staffed by a nurse practitioner, health officers, and nurses and have routine outreach clinics to cater to the needs of the people in rural communities [14][15][16].
Although research addressing NCDs including hypertension has been prioritized, it is still slow and lacks contextualized evidence to aid practice in Tonga [15].According to a recent census report, 77.1% were obese and 37% had hypertension [17].Health education and programs for prevention are mainly run through the Ministry of Health with a priority of addressing NCDs [14].This includes work on policy, food security, health care access, and the availability of health workers to improve efforts toward NCDs [15,16].
The Tonga STEPS Surveys revealed an increase in the prevalence of hypertension from 27.6% in 2014 to 37.1% in 2017 [17].However, little is known about the prevalence of undiagnosed hypertension and its risk factors [3].The purpose of this study was to explore the prevalence and factors associated with undiagnosed hypertension among Tongan adults.Findings from this study could help to inform the Ministry of Health of Tonga in addressing prevention of undiagnosed hypertension and the promotion of awareness programs.

Study area
This cross-sectional study was done with a household survey that was completed from February to March 2023.The catchment area of the Nukunuku Health Center was chosen because it is located inland far from the western coastal area that experienced damage from a tsunami in 2022.Therefore, we thought the catchment area of Nukunuku health center is least likely to be impacted by the tsunami compared to other catchment areas.Additionally, it is 11.5 km away from the main hospital [14].It contains six villages with a total population of 4190 people and 940 households [17].Operational staff includes one nurse practitioner, two NCD nurses, and two reproductive health nurses.Daily, the health center is open for outpatient consultations from nine in the morning until five in the evening.Common prescription medications are also available except for the NCD medications which are often available from the main hospital [14].

Participants
Households were selected with the help of town officers in the villages and health care workers at the Nukunuku Health Center to ensure that the selection met the criteria for inclusion in the study.Upon visiting households, two participants from the same household were invited to join this study, who were: (1) adults aged 18-65 years old; (2) residing in the villages for at least six months; (3) not pregnant; and (4) freely consenting to join the study.From each of the six villages, 40 households were included, accounting for 480 participants that had consented.Among the 480 participants, 473 were retained in the final analysis after excluding 7 participants who turned out not to meet the inclusion criteria.

Variables and measurements
The outcome variable for this study was undiagnosed hypertension as individuals with hypertension who have never been told by a health professional that they have hypertension.Study participants with undiagnosed hypertension were defined as those who answered "No" to the question, "Have you ever been told by a doctor, nurse, or other health workers that you have raised blood pressure?"and showed hypertension during the survey.The operational blood pressure reading was based on the WHO's definition of blood pressure reading for hypertension: blood pressure reading of systolic blood pressure ≥ 140 mmHg and/or diastolic blood pressure ≥ 90 mmHg [1].According to the WHO's protocol [18][19][20], blood pressure was measured twice during the household visit with a duration of at least 5-10 min apart.The averages of both measurements were used in the current study.The measurements were taken using an Omron 7120 digital arm sphygmomanometer with cuff sizes of 22-32 cm and 32-42 cm (Kyoto, Japan).Prior to the survey, calibration of the digital sphygmomanometers was done at the health center when testing the electronic questionnaire with 10 volunteers who visited the health center.
A total of 21 predictor variables were chosen based on similar studies that assessed factors associated with undiagnosed hypertension [4][5][6][7][8][10][11][12].In the current study, the questionnaire addressed these 21 predictor variables, which were self-reported during the household visit interviews.The predictor variables included age, sex, educational level, main source of income, health insurance status, ever having blood pressure measured by a health care professional, knowing common signs of hypertension, experiencing common symptoms, having visited a health center, knowing whether a health checkup is done at a health center, and feelings of difficulty or ease with travelling to the health center.Variables also included whether someone close to the interviewee has hypertension, whether someone close to the interviewee has other NCDs combined as cue to action, knowing the causes of hypertension, risk of cardiovascular disease with hypertension, and awareness of the risks of developing hypertension.The last part of the predictor variables included physical activity, consumption of fruits, consumption of vegetables, salt consumption, smoking, and consumption of alcohol.

Data collection
Data were collected by trained surveyors (registered nurses) using an iPad with the electronic household questionnaire.The questionnaire (Additional file 1) was developed using the Epi Info 7 software and the STEPS Survey questionnaire as validation tool and guidance for the current study.Once households were identified, explanation sheets and consent forms were given, and at least 2-3 days later, surveyors visited to conduct the survey.

Sample size
The sample size was calculated using a formula for logistic regression, n > 10 m, where n is the smaller number of subjects with or without the outcome, and m is the number of predictor variables [21].As 15 predictor variables that would be potentially included in a multivariate model, n should be over 150.Assuming that the prevalence of undiagnosed hypertension would be 40% [4][5][6][7][8][10][11][12], at least 375 participants were needed.Assuming that the participation rate would be 80%, based on the participation rate of 85% in the Tonga STEPS Survey 2017 [16], at least 469 eligible people should have been invited.The sample size was increased to 480 people to protect against uncertainties in the sample size estimations.Since there were no similar studies or pilot studies done in Tonga, it was difficult to estimate the sample size for the current study.Therefore, sample size was slightly increased to prevent type II error, and to protect against uncertainties in the sample size estimations.Among the uncertainties special attention was given to the assumed prevalence of undiagnosed hypertension as the prevalence of undiagnosed hypertension varies across settings [9].

Statistical analyses
Descriptive analyses were done to summarize the number and proportion of participants for each category.Bivariate analyses were performed to assess the association between the outcome variable and predictor variables using Fisher's exact test.Multivariate analyses were conducted using logistic regression.
Only the predictor variables that were statistically significant in Fisher's exact test were included in the multivariate analysis.A p-value < 0.05 was considered statistically significant.In the multivariate analyses, multi-level modeling was used to account for the hierarchical structure of the data: individuals were nested within households, and households were nested within villages.All the analyses were performed using the EZR software version 1.60 [22].

Ethical consideration
Ethical approval was obtained from the National Health Ethics and Research Committee in Tonga, (Approval number 20221212) and from the Ethics Committee of the University of the Ryukyus for Medical and Health Research Involving Human Subjects (Approval number 2063).Participants were given both an explanation and written consent form, which was obtained before the data collection.Individuals in the current study who had shown undiagnosed hypertension were advised and referred to the health center for follow-up and treatment.

General characteristics of participants
Approximately half of the participants (50.9%) were females (Table 1).The median age was 39 (inter-quartile range: 26 to 51) years.The most common education level was secondary school (70.2%), followed by technical vocational educational training (19.0%).The main source of income was civil servants (40.6%), followed by fishing (20.3%), farming (14.6%), and handicrafts (12.9%).Only 12.1% of participants reported having health insurance, and the main reason for not having it was not being able to afford it, followed closely by a lack of information.

Blood pressure reading and health conditions
The majority had their blood pressure measured by health professionals at some point (88.6%), and 71.3% of them had measurements done in 2023 or 2022 (Table 2).These readings were mostly performed in the main hospital (50.4%) and church screening (20.8%), and only 15.3% of the readings were performed at Nukunuku Health Center.A previous diagnosis of hypertension was recorded for 23% of participants, and 67.8% had been diagnosed between 2023 and 2022.The place of diagnosis was mainly the main hospital (43.1%), and only 19.3% had been diagnosed at Nukunuku Health Center.
Only 19.2% had any chronic diseases that were confirmed at a healthcare facility, and 49.5% of them were taking medications for hypertension followed by diabetes.Most of the participants (71.9%) were not aware of the common signs of hypertension.Blood pressure readings during the survey revealed that the mean systolic blood pressure was 131 mmHg, and the mean diastolic blood pressure was 83 mmHg.The prevalence of undiagnosed hypertension was 22.4% (95% confidence interval (CI) 18.7% to 26.4%).

Perception on difficulty of visiting Nukunuku health center
Most participants (67.4%) had visited the Nukunuku Health Center at some point (Table 3), especially within the past two years (75.3%).The top reason for visiting the health center was sickness (44.2%), and only 12.2% visited it for a health check-up.Among those who had visited the health center, nearly half (43.8%) were not satisfied with the services provided there.The most common reason for not being satisfied was the long waiting time (54.3%).
Nearly half of participants (44.8%) did not know whether one can have a health check-up at the health center.Regarding the perceived difficulty or ease of visiting the health center, most participants (73.2%) perceived that it was "not difficult but not easy." For most participants (79.3%), transport was available to travel to the health center.

Health check-up and knowledge on hypertension
Over half of the participants (53.5%) had someone close to them with hypertension, and 53.3% had someone close to them with other NCDs (Table 4).Approximately half of the participants (51.2%) answered that they know a cause of hypertension, and the most reported cause was the consumption of salty food, followed by a lack of physical activity.Furthermore, 58.6% knew that having hypertension puts them at risk of developing cardiovascular diseases, stroke, and diabetes.Nearly half of the participants (49.3%) were aware of the risks of developing hypertension.

Lifestyle behaviors
Approximately 62.5% of the participants were physically active for at least 3 days per week (Table 5), and 8.3% were physically active daily for at least 30 min.Regarding consumption, only 11.4% ate fruits daily, and only 11.4% consumed vegetables daily.Most participants (63.3%) always, often, or sometimes add salt to their food before eating, more than half (58.4%) smoked daily, and 22.4% consumed alcohol.Among alcohol consumers, there were very few habitual drinkers; about 40.6% consumed it a few times per week.

Fisher's exact test
Bivariate analyses using Fisher's exact tests showed that five predictor variables had a statistically significant association (i.e., p-value less than 0.05) with undiagnosed hypertension (Table 6).Hence, variables that were statistically associated were age, not having blood pressure checked before, feelings of difficulty with accessing health care services, cue of action to change behavior (i.e., the existence of someone close to a participant who had hypertension or other NCD), and awareness of developing hypertension.

Results of logistic regression
Multivariate mixed-effects logistic regression analysis showed that three factors were significantly associated with undiagnosed hypertension (Table 7).The first factor was not having blood pressure checked before with odds ratios (OR) as follows; (OR 2.24, 95% CI 1.04-4.77) in comparison to those who did have it a The number of responses is not equal to the total number of participants as seven participants were taking medication for two or more diseases b The number of responses is not equal to the total number of participants as only 179 answered "yes" to the questions about common symptoms

Discussion
The results showed that 22.4% (106/473) of the study population had undiagnosed hypertension.In a global review of hypertension, the denominator used was only the people who had both been diagnosed before and were undiagnosed [9].When applying this to the current study, the percentage of undiagnosed hypertension is 49.3% (106/215).The current results show that undiagnosed hypertension is a significant public health problem that needs to be addressed by the Tongan government.
According to the results, undiagnosed hypertension was significantly associated with three variables: no experience of blood pressure measurement, a lack of awareness of developing hypertension, and age.Participants who never had their blood pressure measured before were significantly more likely to show undiagnosed hypertension compared to those who had it checked recently (33.3% vs. 19.1%).This finding is in line with the statement of the International Society of Hypertension that every adult should know their blood pressure as a starting point to address hypertension [23].In response to this, an international screening campaign for hypertension was launched from 2017 to 2019, which found that 32.0% had never had their blood pressure measured [24].
Additionally, studies in Fiji [7], France [10], the United States [8], and China [25] reported that individuals who had regular visits to their doctors and health centers had more opportunities for regular health check-ups.One of the other documented reasons for this is that hypertension is silent and asymptomatic, so individuals usually do not see the need to regularly visit their healthcare facilities [6,7,11].It is startling that even though simple inexpensive regular health check-ups can contribute to early detection, prevention, and management of hypertension, a "No answer" was excluded b "Do not know" was excluded it is not being utilized well in many low-and middleincome countries.As suggested in many reports by the WHO, each country needs to contextualize its response plans and strategies to address hypertension.Tonga is a small island country with a population of 100,180 people and faces high mortality from coronary heart diseases.Early detection, prevention, and management of hypertension can contribute well to the improvement of cardiovascular diseases.
The second factor was a lack of awareness about developing hypertension.This finding is in line with a review examining hypertension in low-and middle-income countries, which stated that the first barrier to addressing hypertension is poor awareness [26].In the current study, of the participants who had visited the health center, 78.7% answered that they had not seen or could not remember seeing, hearing, or receiving health promotion activities for hypertension while visiting the center.In previous evaluation studies in other countries on the effectiveness of awareness campaigns, the campaigns had positive impacts on early detection, prevention, and treatment of hypertension [25][26][27][28][29].It is also evident from previous studies and reviews in low-or middle-income countries have limitations in terms of both human and financial resources to combat hypertension.
The last associated factor for undiagnosed hypertension in the current study was greater age.One of the reasons for older adults having undiagnosed hypertension is that as one ages, arterial stiffness contributes to increases in blood pressure [3].In studies in France, Ethiopia, and Fiji, reasons given for middle-aged adults (30-50 years) having undiagnosed is the assumption that they are healthy, so they do not go for regular check-ups [7,10,11,[23][24][25].The trend of undiagnosed hypertensive Tongan adults from the current study is consistent with this finding of adults aged 30-49 years showing higher prevalence of hypertension.
Only 12.1% of the participants had health insurance (Table 1).The result is consistent with that of the Tonga STEPS Survey 2017 that reported that only 11.3% of the survey respondents had health insurance [16].Currently, in Tonga, health care is free for all citizens [15] and could be the reason why Tongan people do not include health insurance in their budget.In previous studies, having health insurance had positive impacts on health outcomes and the frequency of visits to health care providers [30][31][32][33][34][35].Thus, it could be a contributing factor for Tongan adults not having regular health check-ups because of a lack of health insurance scheme, but further research and exploration are needed.According to the present study, the most common reason for not having health insurance was affordability.Therefore, the government should consider providing affordable health insurance to promote regular health check-ups.
In the current study, most diagnoses of hypertension were made at the main hospital (43.1%), followed by church screening (20.8%) and health center (15.3%) (Table 2).It is natural for screening to be done in hospital and health center settings, but it is evident from the current study that church screening can contribute to early diagnosis of hypertension.Although screenings are done, more than half of participants (52.4%) were not aware of their own blood pressure readings and about the next steps to take.
This study had two main limitations.First, this study used a non-probability sampling method, so sampling bias is a concern.However, the impact of the bias on the study findings would be small as 240 out of a total of 940 households were covered by the study.Furthermore, no household rejected the invitation to study.Second, this study was limited to one health-center zone on the main island, so the applicability of the findings to wider areas of Tonga would be a concern.However, we believe that the findings of this study can be applied to other healthcenter zones on the main island as socio-economic situations are not greatly different between zones.

Recommendations
Three main recommendations from this study, firstly is the promotion and advocation of regular health checkups.As shown in studies in other countries like Japan [31], China [36], and Vietnam [37], people who have had regular health check-ups have earlier detection and better management of hypertension.Secondly, government and ministry of health Tonga should empower and streamline regular health check-up [25][26][27] process and sharing of information is recommended to enhance continuity of health care [28,29,[33][34][35][36]. Lastly is to encourage young and older adults to participate and raise awareness on hypertension since they are the age group mostly associated in this current study with undiagnosed hypertension.
Considering the limitations of the current study, we propose further research using a randomly selected population sample from the wider areas of Tonga, so that we confirm the findings of the current study.

Conclusion
The prevalence of undiagnosed hypertension is substantial among the Tongan adults examined in this study, indicating a need to address this issue.Findings from this study implicate a need to revisit public health measures on prevention, early detection, and management of hypertension.For instance, having more consistent health check-up opportunities and awareness campaigns using the young and older adults as target groups.Undiagnosed hypertension was significantly associated with not having blood pressure measured, a lack of awareness about hypertension and age groups of middle-aged and older adults.The Tongan government should provide people with more opportunities to have their blood pressure measured and to improve awareness.This could contribute to the fight to control the burden of premature deaths in the country.

Table 1
Demographic and socio-economic characteristics of participants

Table 2
Table showing blood pressure and health conditions of participants

Table 3
Table showing the use and perception of Nukunuku Health Center (nearest healthcare facility) checked in 2022 and 2023.The second factor was not being aware about developing hypertension (OR 1.81, 95% CI 1.08-3.03) in comparison to those who were aware.Last factor was ages of 30-49 years (OR:3.58,CI 1.85-6.95)and 50-65 years (OR 3.38, 95% CI 1.63-6.99),which were associated with higher chances of

Table 4
Table showing cues to the action of health check-ups, knowledge about and perception of hypertensionHas someone close to you had hypertension and therefore visits the health center sometimes?

Table 5
Table showing lifestyle behaviors of participants

Table 6
Table showing bivariate analysis: Fisher's exact test on association of undiagnosed hypertension with predictor variables

Table 7
Logistic regression analysis of factors associated with undiagnosed hypertension a CI: confidence interval b BP: blood pressure c "No" answers were excluded d NCD: non-communicable disease